[gdb/testsuite] Fix gdb.cp/psymtab-parameter.exp with -readnow

[gdb/testsuite] Fix gdb.cp/psymtab-parameter.exp with -readnow

When running test-case gdb.cp/psymtab-parameter.exp with target board readnow,
we run into:
...
FAIL: gdb.cp/psymtab-parameter.exp: maintenance info symtabs
...

The FAIL is expected, as mentioned in the comment:
...
 # The goal is to keep the CU (Compilation Unit) unexpanded.  It would be
 # rather XFAIL than FAIL here.  For example -readnow breaks it.
 gdb_test_no_output "maintenance info symtabs"
...

Fix the FAIL by skipping the command for -readnow.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-10-27  Tom de Vries  <tdevries@suse.de>

	* gdb.cp/psymtab-parameter.exp: Don't expect unexpanded CU for
	-readnow.